Draft control system for shafttype furnaces



B. F. BORGEL 3,021,126

DRAFT CONTROL SYSTEM FOR SHAFT-TYPE FURNACES Feb. 13, 1962 Filed Sept. 9, 1960 INVENTOR TM/MAX @M191 L ATTORNEY 3,021,126 DRAFT CONTROL SYSTEM FR SHAFT- TYPE FURNACES Bernard F. Borgel, Hoyt Lakes, Minn., assigner to Erie lVlining Company, a corporation of Minnesota Filed Sept. 9, 196i). Ser., No. 54,882 4 Claims. (El. 263-29) The present invention relates generally to a draft control system for shaft-type furnaces, and more particularly to an electro-pneumatic system for detecting and automatically preventing incipient furnace blows.

In shaft-type furnaces commonly used today for the induration of pellets-or equivalent small fluent bodiesthe major drawback is presented of the frequent and regular occurrence of undesirable furnace blows. These furnace blows--which may occur as often as 1GO times a day for a given furnaceoccur either by a subsidence of the furnace burden at one place or by a buildeup in the furnace pressure which eventually erupts at a given place in the furnace bed causing an explosion of dust particles and pellets therefrom. These eruptions not only cause hot pellets to be thrown about within the furnace resulting in frequent escape of the pellets through furnace openings, but also the furnace blows ll the immediate area with dust and chip particles. These furnace blows or eruptions cause interruption of the furnace operation and often necessitate a shutting olf of the furnace blower for periods ranging up to several minutes. In most cases it is possible-several minutes after such eruptions-to again feed green (i.e., untreated) pellets into the furnace as before, but if the eruption has been too serious, it is necessary to charge the furnace with previously red or partially tired pellets.

The primary object of the present invention is to provide means for forecasting when a furnace blow will occure and for automatically reducing the amount of air supplied to the furnace for a short period of time so that the occurrence of the furnace blow will be avoided in a positive manner. As a result of the present invention the furnaces may be operated at a higher rate without the danger of furnace blows, whereby the capacity of the furnace is greatly increased (from approximately 40 long tons per hour per furnace to an average of better than 50 long tons per hour). Furnace maintenance costs have been materially alleviated as a result of the use of the present invention because of the reduction of the amount of material thrown out of the furnaces which would damage furnace doors, the charging mechanism, and the like.

A more specific object of the present invention is to provide an electro-pneumatic damper control system for automatically reducing the air flow to the furnace blower upon the detection of a reduction in the furnace hood suction pressure which would result in a furnace blow, said damper control system including draft responsive gauge means operable to actuate a three way solenoid valve to switch the damper control air supply from a high-pressure line to a reduced-pressure line, said damper control system also including ow-set speed control means for delaying the restoration of normal air to the furnace for a relatively short given period of time.

Other objects and advantages of the present invention will become more apparent from a study of the following specification when considered in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of the furnace draft control system, the pneumatic connections between the elements being shown by solid lines and the electrical connections being shown by Vdashed lines; and

FIG. 2 is a schematic electrical wiring diagram of the damper is caused to be fully open.

anice system, in which solid lines are used to show electrical connections.

Referring rst more particularly to FIG. 1, the standard shaft-type indurating furnace 10 is provided with reciprocatory continuous-belt conveying means 11 at the upper end thereof for evenly distributing layers of green pellets to be treated upon the stockline 12. The column of pellets 13 travels by gravity downwardly through the shaft furnace for discharge through the discharge legs 14, 15. Rotary chunk breaker means 16 are provided adjacent the lower portion of the furnace shaft to break up any chunks of pells which may have been formed in the shaft. A fuel-air mixture is burned by the burners 17 in the combustion chamber 18 and the hot combustion gases are introduced into the furnace shaft through the openings 19. In a known manner, the heated air passes upwardly through the pellets above the openings 19 and is drawn upwardly through the furnace hood portion 10a through the exhaust conduit 20 by the suction fan 21. Cooling air is blown into the lower portion of the shaft furnace through the air louvers 22 and is drawn upwardly by the suction fan 21 to cool the pellets as they pass downwardly below the level of the openings 19 so that by the time the pellets are discharged through the discharge legs 14, 1S the temperature of the pellets is materially reduced. Air is supplied from the main air supply 23 to the air louvers 22 through the conduit 24 which contains in series the damper 25 and the furnace blower 26. A certain portion of the air is fed to the fuel supply means 27 for mixture with the fuel, the resultant fuel-air mixture being then fed to the burners of the furnace combustion chamber.

According to the present invention the damper operator is automatically pneumatically controlled to prevent incipient furnace blows. To this end, the following electro-pneumatic damper control system is provided.

Damper control air is fed from the control air supply 30 and through the manual controller valve 31 to one inlet opening 32 of the three-way solenoid-operated valve of the electro-pneumatic relay 3. Damper control air is also fed through the manually adjustable reducing valve 7 to the second inlet opening 33 of the three-Way solenoid-operated valve. The damper control air passes from the outlet 34 of the three-way solenoid valve through the flow-set speed control valve 2 and through the solenoidcontrolled electro-pneumatic relay 35 to the damper operator 25a. The damper operator 25a is a Hagan pistontype pair cy'inder mechanically connected to the blower damper in such a manner that when Zero control air pressure exists on the piston, the damper is in the fully closed position. When a certain control air pressure (for example, 12.5 pounds) is applied to the piston, the Between open and closed positions, the degree of opening of the damper is substantially directly proportional to the control air pressure applied to the damper operator.

As will be described in greater detail below, the operation of the solenoid-controlled electro-pneumatic relay 3 is electrically controlled-as illustrated schematically by the dashed-line electrical connection 'S7-by the draft responsive pressure gauge 1 which is continuously pneumatically operable through conduit 38 as a function of the suction pressure, relative to atmospheric pressure, which exists in the furnace hood 10a. As will be described in greater detail below with reference to FIG. 2, the draft responsive gauge includes movable and stationary switch contacts which may be adjustably set to operate when the pointer of the gauge has been displaced a predetermined amount relative to its origin. The unit is so arranged that at an adjustable preset point of draft' (i.e., suction), the electrical contacts will open upon lowering of the draft and close (at the predetermined gauge point) upon raising of the draft.

The electro-pneumatic relay 35 comprises a solenoid valve electrically operable through the electrical connection 40 as a function of the furnace gas temperature as determined by the thermal measuring device 41. The electro-pneumatic relay 35 operates to reduce the control air pressure on the damper operator to zero (thus shutting off air to the furnace) when the top gas temperature exceeds a predetermined temperature setting. Thus a basic safeguard is presented against serious overheating of the furnace hood, dust collector, and exhaust gas fans.

Referring now to the electrical schematic diagram of FIG. 2, the switch contacts 43, 44 of the draft responsive gauge 1 are connected in series with the solenoid 45 of the auxiliary electrical relay 4 across the main power lines L1 and L2. The auxiliary relay 45 has switch contacts 46, 47 operable by the solenoid 45 and connected in series with the manually operable selector switch 48 and the solenoid 49 of the electro-pneumatic relay 3. The series circuit consisting of the auxiliary relay switch 46, 47, selector switch 48 and solenoid 49 is also connected across the main power lines L1, L2. Pilot light 6 is connected in parallel across the selector switch 48 and the electro-pneumatic relay 3. In the normal operating condition of the draft responsive gauge 1, with suicient suction existing in the furnace hood tn, the contacts 43, 44 of the gauge 1 are closed, solenoid 45 is energized, and relay contacts 46, 47 are maintained open. When the furnace suction decreases, however, to the point predetermined for the draft pressure gauge 1, the switch contacts 43, 44 are opened, solenoid 45 is de-energized, and switch contacts 46, 47 close to energize the pilot light 6 and the electro-pneumatic relay 3.

The manually adjustable reducing valve 7 is initially preset to establish the control air pressure which is to be impressed on the damper operator when the draft responsive gauge operates to energize solenoid 45 and thus effect communication between inlet 33 and outlet 34. The normal control air pressure impressed on the damper operator through the manual controller valve 31 (when inlet 32 communicates with outlet 34) is approximately 12.5 pounds, resulting in a damper position which produces approximately 24,000 cubic feet per minute of air from the main blower. Since the opening of the damper is directly proportional to the control air pressure impressed on the damper operator 25a, presetting of the manually adjustable valve 7 for a 5 pound control air pressure would result-upon operation of the switch contacts of draft responsive gauge 1 and operation of the electro-pneumatic relay 3-in a decrease of air admitted to the furnace from 24,000 c.f.m. to 9600 c.f.m.

It is important to note that the flow-set speed control valve 2 is provided with an adjustable-orifice by-pass path 50. Since the damp operator 25a is so arranged that the damper on the main air blower is closed (resulting in zero air flow to the furnace) when the impressed control air pressure is zero, and is wide open (resulting in maximum air flow to the furnace) when the impressed control air pressure is 12.5 p.s.i., the speed control valve 2 functions as a check valve to allow the rapid diminution of control air pressure impressed on the damper operator when required by the operation of the draft responsive gauge 1, and the gradual restoration of the original control air pressure on the damper operator 25a when the furnace blow has ceased. Thus the flow-set speed control valve 2 with adjustable-orifice by-pass 50 permits full air ow in one direction and a predetermined restricted air flow in the opposite direction. Consequently, an immediate decrease of air ow to the furnace upon indications of a blow is permitted, but the restoration of normal air to the furnace is delayed for a period of approximately 45 seconds. While in a prototype installation a standard apper type check valve proved to be operable, because of the lack of adjustability of the rate of return of such a device, the use of a Hanna-type valve has been found to be more desirable.

The operation of the damper control system may now be readily described.

Assuming that the shaft furnace is operating with proper hood suction, control air having a pressure of approximately 12.5 pounds (as determined by the manual controller valve 31) is fed to the damper operator 25a through the inlet 32 and outlet 34 of the electro-pneumatic relay 3, through the flow-set speed control valve Z, and through the electro-pneumatic relay 3S. The damper operator 25a will then cause the damper to be fully open so that air will be fed to the furnace by the blower at a rate of approximately 24,000 cubic feet per minute.

Upon the occurrence of a lowering of the furnace hood suction (as detected by the orifice 38a of line 38 in the hood 10a) prior to an incipient furnace flow, the pointer of the draft control gauge 1 will be displaced to a point to open the switch contacts 43, 44 thereof. Solenoid 45 of the auxiliary electrical relay 45 will then be deenergized to cause closing of the switch contacts 46, 47 and energization of the solenoid 49 of the electro-pneumatic relay 3, whereby the control air supply impulses will be switched from the normal pressure set by the manual controller valve 31 (on the order of l5 pounds) to the reduced pressure (on the order of 5 pounds) set by the manually adjustable valve 7, thus resulting in partial closure of the blower air vanes and decrease of the air flow to the furnace. Upon restoration of the furnace hood draft or suction above the preset control position of the draft responsive gauge 1 (achieved as a result of the lowered quantity of blower air fed to the louvers 22 relative to the suction pressure developed by fan 21), the switch contacts 43, 44 are closed, the solenoid 45 is energized, switch contacts 46, 47 are opened and the electro-pneumatic relay 49 is de-energized to return original control air pressure to the damper operator 25, thus restoring normal air ow to the furnace. The restoration time for air flow is approximately 45 seconds as a result of the delay effect of the flow-set speed control valve 2.

While in accordance with the provisions of the patent statutes I have illustrated and described the -best forms and embodiments of my invention as now known to me, it will be apparent to those skilled in the art that other changes and embodiments may be made in the apparatus described without deviating from the invention as set forth in the following claims.

l claim:

1. A draft control system for a pellet indurating furnace of the generally vertical shaft type composed of, in series, an upper hood portion, an intermediate portion adapted to have a stock line region therein, and a lower portion, said furnace having blower means adapted for supplying air into the lower portion of the furnace said blower means being controlled by a blower damper, means for supplying air into the lower portion of the furnace, means adapted for supplying heated gases into said intermediate portion of the furnace below the stockline thereof, and suction fan exhaust means for withdrawing gases from the upper hood portion of the furnace, said draft control system comprising a pneumatically-controlled damper operator controlling the degree of opening of the blower damper as a direct function of the quantity of control air supplied to said damper operator, a source of damper control air, a three-way solenoid-operated valve having an outlet and first and second inlets, a manual controller valve having an inlet connected to said control air source and an outlet connected to the first inlet of said three-way valve, a manually adjustable reducing valve having an inlet connected to said control air source and an outlet connected to said second inlet of said three-way solenoid valve, a flow-set speed control valve having an inlet connected to the outlet of said threeway solenoid valve and an outlet connected to said damper operator, said liow-set speed control valve including a restricted oriiice by-pass passage, the outlet of said three-way solenoid valve being normally solely in communication with the irst inlet thereof to feed a predetermined quantity of damper control air from said controller valve to said damper operator through said flow-set speed control valve, said predetermined quantity of air causing opening of said damper to a substantially fully open iirst position, and electrical means responsive to decreases in furnace hood suction pressure below a predetermined value for actuating said three-way solenoid valve to cause the outlet thereof to be solely in communication with the second inlet thereof to supply damper control air from said reducing valve to said damper operator through said flow-set speed control valve, said reducing valve being set to provide control air to said damper operator of lower pressure than that provided by said controller valve so that said damper will be displaced to a partially-open second position, said electrical means being automatically operable upon return of the hood suction pressure to said predetermined value to actuate said three-way valve to cause the outlet thereof to again communicate with said rst inlet, said ow-set speed control valve causing delay of the control air pressure supplied from said controller valve to said damper operator to delay the return of the damper to its fully open position.

2. Apparatus as defined in claim 1 and further including second solenoid-operated valve means connected in series intermediate the outlet of said ow-set speed control valve and said damper operator, said second solenoid-operated valve means being operable when the temperature of the furnace hood exceeds a predetermined value to interrupt the passage of damper control air to said damper operator and thus cause said blower damper to become completely closed.

3. Apparatus as defined in claim 1 wherein said electrical means for actuating said three-Way solenoid valve includes a pressure responsive gauge connected pneurnatically to said furnace hood and including electrical switch contacts which are actuated when the indicator of said gauge is positioned at a predetermined position reiative to the gauge origin.

4. Apparatus as defined in claim 3 wherein said electrical means includes an electrical circuit comprising a pair of power supply lines, and an auxiliary relay including switch contacts and a solenoid controlling the operation of said auxiliary relay switch contacts, said relay contacts being normally closed when said auxiliary relay solenoid is energized, said auxiliary relay solenoid being connected in series with the pressure gauge switch contacts across said power lines and said auxiliary relay switch contacts being connected in series with the solenoid of said three-way valve across said power lines, whereby opening of the switch contacts of said pressure responsive gauge causes de-energization of said auxiliary electrical relay, closure of the switch thereof, and energization of solenoid of said three-way solenoid valve.

References Cited in the file of this patent UNITED STATES PATENTS 2,814,479 Leone Nov. 16, 1957 

